LM317SSLASHNOPB datasheet(12/40 Pages) TI1

12 / 40 page

LM117, LM317A, LM317-N

SNVS774O – MAY 2004 – REVISED JANUARY 2014

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Heatsink Requirements

The LM317-N regulators have internal thermal shutdown to protect the device from over-heating. Under all

operating conditions, the junction temperature of the LM317-N should not exceed the rated maximum junction

temperature (TJ) of 150°C for the LM117, or 125°C for the LM317A and LM317-N. A heatsink may be required

depending on the maximum device power dissipation and the maximum ambient temperature of the application.

To determine if a heatsink is needed, the power dissipated by the regulator, PD, must be calculated:

PD = ((VIN − VOUT) × IL) + (VIN × IG)

(2)

Figure 25 shows the voltage and currents which are present in the circuit.

The next parameter which must be calculated is the maximum allowable temperature rise, TR(MAX):

TR(MAX) = TJ(MAX) − TA(MAX)

(3)

where TJ(MAX) is the maximum allowable junction temperature (150°C for the LM117, or 125°C for the

LM317A/LM317-N), and TA(MAX) is the maximum ambient temperature which will be encountered in the

application.

Using the calculated values for TR(MAX) and PD, the maximum allowable value for the junction-to-ambient thermal

resistance (

θJA) can be calculated:

θJA = (TR(MAX) / PD)

(4)

Figure 25. Power Dissipation Diagram

If the calculated maximum allowable thermal resistance is higher than the actual package rating, then no

additional work is needed. If the calculated maximum allowable thermal resistance is lower than the actual

package rating either the power dissipation (PD) needs to be reduced, the maximum ambient temperature TA(MAX)

needs to be reduced, the thermal resistance (

θJA) must be lowered by adding a heatsink, or some combination of

these.

If a heatsink is needed, the value can be calculated from the formula:

θHA ≤ (θJA - (θCH + θJC))

(5)

where (

θCH is the thermal resistance of the contact area between the device case and the heatsink surface, and

θJC is thermal resistance from the junction of the die to surface of the package case.

When a value for

θ(H−A) is found using the equation shown, a heatsink must be selected that has a value that is

less than, or equal to, this number.

The

θ(H−A) rating is specified numerically by the heatsink manufacturer in the catalog, or shown in a curve that

plots temperature rise vs power dissipation for the heatsink.

Heatsinking Surface Mount Packages

The TO-263 (KTT), SOT-223 (DCY) and TO-252 (NDP) packages use a copper plane on the PCB and the PCB

itself as a heatsink. To optimize the heat sinking ability of the plane and PCB, solder the tab of the package to

the plane.

Heatsinking the SOT-223 (DCY) Package

Figure 26 and Figure 27 show the information for the SOT-223 package. Figure 27 assumes a

θ(J−A) of 74°C/W

for 1 ounce copper and 51°C/W for 2 ounce copper and a maximum junction temperature of 125°C. Please see

AN-1028 (literature number SNVA036) for thermal enhancement techniques to be used with SOT-223 and TO-

252 packages.

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